1. Field of the Invention
The present invention relates to communication systems. More specifically, the present invention relates to differential detection of multiple phase shift keyed signals.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
As is well known in the art, differential detection schemes utilize a phase reference provided by a previously transmitted symbol to establish a phase reference for the demodulation of a currently received symbol. Differential detection is an attractive alternative to coherent detection in applications where simplicity and robustness of implementation take precedence over optimal system performance. In addition, differential detection may be the only alternative in severely degraded transmission environments, e.g., multipath fading channels, in which acquisition and tracking of coherent demodulation reference signals is difficult if not impossible.
In the past, differential detection of multiple-phase-shift keying (MPSK) has been accomplished by comparing the received phase in a given symbol interval with that in the previous symbol interval and making a multilevel decision on the difference between these two phases. See "Investigation of Digital Data Communication Systems," by J. G. Lawton, Report No. UA-1420-S-1, Cornell Aeronautical Laboratory, Inc. Buffalo, N.Y., Jan. 3, 1961. (Also available as ASTIA Document No. 256 584.) In Telecommunication Systems Engineering, published in 1973 by Prentice-Hall of Englewood Cliffs, N.J., at page 240-252, authored by W. C. Lindsey and M. K. Simon provide an implementation of such a receiver and the analysis of its error rate performance on an additive white Gaussian noise (AWGN) channel. In arriving at the results in these works, the assumption was made that the received carrier reference phase is constant over at least two symbol intervals so that its effect on the decision process cancels out when the above-mentioned phase difference is taken. This assumption is critical to the analysis but is also realistic in many practical applications. Also, since the information is carried in the difference between adjacent received phases, the input information must be differentially encoded before transmission over the channel.
Although differential detection eliminates the need for carrier acquisition and tracking in the receiver, it suffers from a performance penalty (additional required signal-to-noise ratio (SNR) at a given bit error rate) when compared with ideal (perfect carrier phase reference) coherent detection. The amount of this performance penalty increases with the number of phases, M, and is significant for M.gtoreq.4. For example, at a bit error probability P.sub.b =10.sup.-5, differentially detected binary phase shift keyed, BPSK, (often abbreviated as DPSK) requires approximately 0.75 dB more bit energy-to-noise ratio (E.sub.b /N.sub.o) than coherently detected BPSK (with differential encoding and decoding). For QPSK (M=4), the difference between differential detection and ideal coherent detection (with differential encoding and decoding), at a bit error probability P.sub.b =10.sup.-5, is about 2.2 dB. Finally, for 8PSK, the corresponding difference in performance between the two is greater than 2.5 dB.
Thus, there is a need in the art for an improvement over the conventional (two symbol observation) differential detection technique so as to recover a portion of the performance lost relative to that of coherent detection while maintaining a simple and robust implementation. That is, there is a need in the art for an improvement in the conventional differential detection scheme with minimal additional complexity.